1. Field of the Invention
This invention relates to a material for forming a metal image, and more specifically, to an improvement in a metal image-forming material capable of forming an image of high contrast and suitable for halftone dots or lines.
2. Description of the Prior Art
Generally, very high contrast lithographic films have heretofore been used as an image-forming material suitable for halftone dots or lines. Lithographic films are photographic materials for forming an image composed of a transparent portion and a completely non-transparent portion, and are characterized in that the edge of the image is clear, and accurate line widths can be obtained. With lithographic films, it is desirable for the image formed to be clearly separated into a transparent portion and a blackened portion with high contrast. When an image is formed using a lithographic film, however, some "rising width" (toe of characteristic curve) exists from the density of the support film to the image area as shown by the characteristic curve thereof, and appears as a fringe at the periphery of the image. Furthermore, since the development of a lithographic film is based on infectious development, the image increases in size and grows as development proceeds. For this reason, development cannot be stopped instantaneously, and even immediately after stopping the development, the development proceeds slightly. This is also considered to be responsible for fringe formation. As a result, images of ideal quality cannot be produced using lithographic films.
Photographic materials composed of a transparent plastic film and a photosensitive resin layer formed thereon were developed in an attempt to solve the problems associated with such lithographic films. These photographic materials can be handled in a bright room which is convenient for processing, and also conserve use of silver. However, these types of photographic materials have the disadvantage that photographic materials of this type cannot be used to obtain high density images, and the size reduction of halftone dots, a property required of so-called "reversal films", is difficult.
Extensive investigations have now been made in an attempt to solve the problems associated with the lithographic films and the photographic materials having a photosensitive resin layer (or photoresist) which have been described hereinabove. This work led to the development of a metal image-forming material composed of a support, a metal image-forming layer formed thereon, and a top layer of a photosensitive resin. In such a metal image-forming material, the adhesion of the photosensitive resin layer to the metal image-forming layer is generally weak, and the photosensitive resin layer peels off partly. Moreover, since the surface of the material is tacky, it sometimes sticks to an image-bearing film used for contact printing, or the surface becomes somewhat soft and weak due to the action of an alkaline etching solution used for development. In particular, pinholes are sometimes caused in the photosensitive resin layer due to the action of a strongly alkaline processing solution used for reduction after the formation of a metal image.